Synthesis of high aspect ratio WO2 nanostructures

Tungsten oxide (WO2) nanorods and nanowires were prepared by the heat treatment of WO2 nanocrystalline powders in the presence of Ar. Nanocrystalline powders produced by a simple water-assisted route at the room temperature were annealed at different temperatures for different durations, which yielded orthorhombic and monoclinic WO2 crystals. Annealing the powders at 700 °C and above resulted in orthorhombic WO2 nanorods/nanowires with an average diameter of 60–70 nm beside the monoclinic WO2 nanocrystalline powders with a diameter of ~5 nm. The lengths of the nanorods increased from several 100 nm up to several 10 µm with increasing temperature while their diameters did not change. With increased length, nanowires became more elastic in nature having a cotton-like fabric. The prepared nanostructures have been characterized by X-ray powder diffraction measurements, scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. An oriented attachment mechanism leading to root growth from a parent structure was proposed. High aspect ratio WO2 nanostructures with an average diameter of 60–70 nm and lengths changing from several 100 nm up to several 10 µm were synthesized by a simple annealing process under atmospheric pressure from WO2 nanoparticles (~ 30 nm) and a possible formation mechanism was proposed.